effect of flow rate on the corrosion products formed on

Int. J. Electrochem. Sci., 10 (2015) 2904 - 2920

International Journal of

ELECTROCHEMICAL SCIENCE

www.electrochemsci.org

Effect of Flow Rate on the Corrosion Products Formed on

Traditional and New Generation API 5L X-70 in a Sour Brine

Environment

A. Cervantes Tobón1,*

, M. Díaz Cruz1, M. A. Domínguez Aguilar

2, J. L. González Velázquez

1.

1 Instituto Politécnico Nacional, Departamento de Ingeniería Metalúrgica, IPN-ESIQIE, U.P. Adolfo

López Mateos, Zacatenco, C.P. 07738, México, D.F., México. 2 Instituto Mexicano del Petróleo, Dirección de Investigación y Posgrado, Eje Central Norte Lázaro

Cárdenas 152, Col. San Bartolo Atepehuacan, C.P. 07730, México D.F., México. *E-mail: [email protected]

Received: 27 November 2014 / Accepted: 3 February 2015 / Published: 24 February 2015

The effect of chemical composition on the nature of the corrosion products formed on pipe steels API

5L X-70 traditional and API 5L X-70 new generation was studied. General corrosion was induced

under different flow rates (1~4 m/s) in a rotating disc electrode immersed in a sour brine solution with

kerosene at 60 °C. Linear polarization resistance indicated that API 5L X-70 NG displayed a higher

corrosion resistance because of the presence of copper, which promoted a greater amount of oxides

than sulfides on surface in comparison to those formed in the traditional pipe steel. Moreover, ferrite

increased (70-95%) and perlite decreased (30-5%) in NG steel to protect steel from further dissolution.

Scanning electron microscopy of traditional steel displayed the presence of a larger amount of

corrosion products with evident porosity and bush shape morphology, whereas corrosion products on

NG pipe steel looked similar but more coherent and both covered most of the steel surfaces. X-ray

diffraction analysis showed that the corrosion products are mainly composed of a mixture of oxides

(maghemite, hematite, magnetite), and sulfides (mackinawite, troilite, pyrite, marcasite and smithite)

only in the immersion tests. The protective function of corrosion products on the API 5L X-70 NG

surface turned out to be better because of the presence of one different phase, triclinic pirite, that seems

more stable than cubic pyrite in traditional steel. The presence of copper in new generation steel in

approximately twice as much compared to the traditional steel, which apparently favored the cathodic

reaction where oxygen is formed. Chromium and nickel delayed anode reaction, ferrite dissolution,

and helps to decrease corrosion rate.

Keywords: Corrosion, polarization resistance, pipeline steel, copper.

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Int. J. Electrochem. Sci., Vol. 10, 2015

2905

1. INTRODUCTION

Hydrogen sulfide (H2S) present in the oil transport pipelines is one of the main agents of

corrosion. In this process atomic hydrogen H+ is formed on pipeline surface; which is adsorbed and

recombine to form H2 which accumulates and produces an increase in pressure and causes the

formation of blisters and/or cracks which can lead to fracture of the material [1]. The evaluation and

control of corrosion of carbon steel in sour media is very important to the petroleum industry, because

this process involved high maintenance costs and equipment replacement. It is also responsible for the

loss of human lives and given the importance of corrosion in public safety have established new

regulations and practices of corrosion control for pipelines transporting liquids and gases because of

the high risk they convey [2,3]. Therefore it is important to know the chemical reactions involved in

the different environments of this industry, the type and stability of the corrosion products formed on

the surface of the pipe and their effect on the corrosion process allowing a better control of the process

reducing risks and costs.

In the literature, several researchers have evaluated the corrosion behavior and film growth by

corrosion products in different media simulating different refining conditions, such as media

containing (Cl + H2S), (NH4 + Cl + H2S ) and (Cl + CN + H2S). Shoesmith et al. [4] and Liu et al., [5]

established that the composition of the films of corrosion products depend on the composition of the

medium and pH [4,5]. In the process of corrosion of carbon steel in the presence of H2S, the corrosion

product formed is mainly mackinawite [6]. However, it has been noted the formation of an oxide film

formed by a combination of mackinawite (Fe (1 + x) S), troilite (FeS) and iron sulfide amorphous [4,5].

Tewari and Campbell proposed that the steel in H2S generated solutions FexSy film. There have been

many studies to determine the composition of the corrosion products and their kinetics of formation

[7,8]. However, few studies have been conducted on the effect of the flow rate on the composition and

morphology of the corrosion products as well as on the corrosion rate on two different micro-alloyed

steels. In this work, the study of electrochemical corrosion of new generation pipeline steels is

introduced. The corrosion of steel API 5L X-70 NG (new generation) with a carbon content of 0.1 %

wt maximum [9] was tested in a brine added with kerosene and 1382.7 ppm of H2S at 60°C. Linear

polarization scans, scanning electron microscopy and X-ray diffraction were applied to determine

corrosion rates to be compared with a steel API 5L X-70 T (traditional) with 0.240 wt % of carbon,

besides morphology and composition of corrosion products.

2. EXPERIMENTAL PROCEDURE

2.1. Test Environment.

The test solution was a brine prepared according to NACE standard 1D-196 [10] with 106.56

g/L NaCl, 4.48 g/L CaCl2 2H2O, 2.06 g/L MgCl2 6H2O and 10% kerosene, 1387 ppm of hydrogen

sulfide (H2S) was added. The pH was 3.9 and the temperature of the solution was 60 ºC. The test


2906

solution was deaerated with nitrogen gas for a period of 30 minutes as stated in the ASTM G59-97

[11] to remove dissolved oxygen.

2.2. Experimental set up.

A double bottom cell made of Pyrex glass heated with hot water was used. Cylindrical tests

specimens were cut off from actual pipes of 11 mm or more of thickness in the longitudinal direction.

The total area exposed of the working electrode was 3.5 cm2 for both static and dynamic tests. The

reference electrode was a saturated calomel electrode (SCE), and two auxiliary electrodes of sintered

graphite rods were used. Before each experiment, the working electrode was wet abraded to 600

silicon carbide paper, cleaned with deionized water and degreased with acetone. All electrochemical

tests were performed on recently clean prepared samples and fresh solutions.

2.3. Hydrodynamic conditions.

The hydrodynamic simulations of flow velocity in the laboratory were carried out in a rotating

cylinder electrode (RCE) made by EG & G PARC, model 616 RDE connected to a

Potentiostat/Galvanostat. The corrosion rate of the system was evaluated at different electrode rotation

rates. The working electrode rotational speeds used in this study were varied from 0 to 6500 rpm (3.74

m/s), with increments of 500 rpm. The selection of these ranges were based on the conditions

commonly observed at industrial facilities, as well as on the values of the Reynolds numbers (Re)

allowing the validation of the existent hydrodynamic and mass transfer correlations for the RCE. The

Reynolds number is from 0 to a value of 17,464.8157 (at 6500 rpm) this being a fully turbulent regime.

2.4 Immersion tests

To carry out immersion tests coupons 1 cm long x 1 cm high were prepared for both steels

(API 5L X-70 T and NG), which were immersed for a period of 24 hours in a medium of synthetic

brine prepared in agreement to NACE standard ID-196 added with 10% kerosene + 1382.7 ppm H2S

(equivalent to 1 bar pressure) in order to induce corrosion products on both steel surfaces to be

analyzed by scanning electron microscopy (SEM) for morphology and distribution.

2.5. Corrosion rate measurements.

For conducting linear polarization resistance measurements the standard for conducting

potentiodynamic polarization resistance measurements [11] was applied by means of the commercial

software POWER SUIT by using a Potentiostat/Galvanostat of Princeton Applied Research model

263A. The polarization curves were obtained at a scan rate of 0.166 mV/s working in the range of ±

200 mV vs. the open circuit potential (OCP) in reference to SCE. The corrosion rate was obtained as a

function of flow rate for the steels tested in the environment. To ensure reliability of results three tests


2907

were taken for each flow velocity range employed, allowing the system to rest for 5 minutes before

running the test and recording potential and current density for each of the steels used in the

investigation.

2.6. Characterization of corrosion products by SEM.

The surface morphology and composition of the corrosion products formed on the electrode

surfaces of traditional and new generation API X-70 steels was analyzed by a JEOL 6300 SEM

coupled with EDX.

2.6.1 Physical characterization by XRD

X-ray diffraction (XRD) was used to determine the iron phases on API 5L X-70 (T) and X-70

(NG) steels, with a scanned range from 20° to 90° and a step width of 0.02°, using a D8 Focus Bruker

diffractometer with Cu Kα radiation. Further analyses of XRD spectra were carried out by the CreaFit

2.2 DRXW software.

3. RESULTS AND DISCUSSION

3.1. Chemical analysis and metallography

The chemical composition of pipe steels was obtained by arc spectrometry (spark) and they are

shown below in Table 1.

Table 1. Chemical composition of the API 5L X-70 T and X-70 NG steels (wt. %)

Steel C Mn Si P S Cr Cu Ni Fe

API 5L X-70

(T)

0.240 1.081 0.284 0.019 0.021 0.156 0.185 0.088 97.8

API 5L X-70

(NG)

0.048 0.927 0.222 0.004 0.015 0.017 0.279 0.005 98.4

It can be observed that the API steel 5L X-70 NG has a lower carbon content (0.048 wt%) with

respect to the traditional steel (0.240 wt%); it possesses lower amounts of Mn and Si. NG steel also

shows an improved cleanliness with lower contents of phosphorus and sulfur. Regarding P has a very

low quantity compared to the traditional, which is a typical characteristic of these steels in their

manufacturing process. It also has quantities well below the traditional Cr and Ni relative to copper but

the new generation has approximately as much as twice Cu compared with the traditional.


2908

3.2 Microstructure and grain size

Figure 1 shows the micrographs carried out on the steels proposed in this research. The images

were obtained in the optical microscope, they show the presence of pearlite (dark phase) in a ferrite

matrix (white phase). In addition, for the API 5L X-70 NG (b) a slight pearlite banding due to the

manufacturing process of this steel is observed, this is in agreement with similar microstructures

obtained by others researchers [12-14].

Figure 1. Microstructure of (a) API 5L X-70 T and (b) API 5L X-70 NG steels.


2909

Table 2. Quantitative metallography of API 5l X-70 steels along the longitudinal section.

Steel % Ferrite % Pearlite ASTM Grain

API 5L X-70 T 70.51 29.48 10

API 5L X-70 NG 94.86 5.14 10

Table 2 shows that both steels have the same ASTM grain size with a value of 10, which is a

refined grain. With respect to the amount of ferrite of the steel API 5L X-70 NG, it contains more than

traditional steel and therefore would be expected that this steel will be more easily dissolved when

exposed to the corrosive environment, this effect is not observed on the values of the corrosion rates

apparently because of the protective film of copper on NG steel. Likewise, as a result of the lower

amount of perlite, it will not form a larger amount of sulfur compounds as corrosion products, as it is

the case of traditional steel, which tend to be deposited on this phase. Perlite act as cathode by the

presence of cementite and thereby to more efficiently protect the steel surface [15].

3.3 Immersion tests

Figure 2. Micrographs of steel API 5L X-70 T (a) and NG (b) at 100X immersion test after 24h.


2910

Figure 3. Micrographs of steel API 5L X-70 T (a) and NG (b) at 1000X immersion test after 24h.

Figure 2 shows the uniform corrosion product layer formed on both steels surfaces at 100X, It

is observed that steel API 5L X-70 T (a) generated a greater amount of corrosion products than those

formed on new generation steel (b), which may have some effect on the corrosion rate.

At higher magnifications (1000X), it can be appreciated the morphology of the corrosion

products formed on both steels (Figure 3). The traditional API X-70 displayed a coarser surface with a

higher porosity than that deposited on steel API 5L X-70 NG (b). For new generation steel (b) it can be

seen that a primarily uniform layer was formed and the corrosion products were deposited in lower

amount and the whole surface. As mentioned before nature and morphology may affect the corrosion

rate behavior of these steels.

3.4 Linear polarization resistance and corrosion rates

3.4.1 Corrosion potentials for pipe steels as function of flow rates in sour brine at 60 °C.

Figure 4 shows the corrosion potential behavior as a function of the rotation speed with the

addition of H2S at 60 °C. It is observed that both steels started from an active state, one more (API 5L

X-70 NG) than the other (API 5L X-70 T), since both have a clean surface free of corrosion products;

then the two steels pass into a less active state with a tendency to become independent of the rotational

speed, which it may indicate a steady growth of corrosion products. Note that the least active (~ 20

mV) to the end of the test is the API 5L X-70 T. Results show that the corrosion tendency is slightly

higher for the API 5L X-70 NG; in this case activity indicated that corrosion products begin to form

and evolved uniformly throughout the flow velocity range employed, therefore it is not further seen a

significant increase in the tendency to corrosion due to the corrosion potential does not increase

greatly, so it could be assumed that the corrosion products formed are more stable and uniform in the

NG steel.


2911

Figure 4. Corrosion potential of API 5L X-70 T and API 5L X-70 NG as a function of flow rate for

the steels in sour brine with kerosene added with H2S at 60 °C.

3.4.2 Comparison of the corrosion rates as function of flow rates for steels API 5L X-70 T and API 5L

X-70 NG in sour brine at 60 °C.

Figure 5. Corrosion rate of API 5L X-70 T and API 5L X-70 NG as a function of flow rate for the

steels in sour brine with kerosene added with H2S at 60 °C.


2912

Figure 5 shows the results for the corrosion rate of pipe steels in sour brine with kerosene and

H2S at 60 °C. From these results, it was observed a high corrosion rate from the very beginning for

API 5L X-70 T (8 mm/y )and API 5L X-70 NG (9 mm/y), later corrosion rate increased with the

increase in the flow rate to reach a top of about 15 mm/y . The variation of the corrosion rate with the

flow velocity is generally attributed to changes in the corrosion mechanism [16]. These steels showed

a similar behavior, which suggests that the corrosion products formed on surface are more stable as

there is not significant detachment of the corrosion products. The API 5L X-70 T tested between 6000

and 6500 rpm shows an increase in the corrosion rate, which indicated that there was a detachment of

the corrosion products formed as a result of fluid turbulence and consequently the wall shear stresses

diminished the thickness of this layer [17], which led to an increase of the corrosion rate, so this steel

(T) may not be quite stable or have a good adherence when compared to the NG steel. In the case of

API 5L X-70 NG, it was observed that within 5500 to 6500 rpm, the corrosion rate remains almost

constant due to the continuous formation of corrosion products on steel, besides its corrosion rate is

slightly lower compared to API 5L X-70 T.

API 5L X-70 NG displayed a higher corrosion resistance than API 5L X-70 T. In this case the

microstructure apparently had no effect on the corrosion rate. The latter steel would expect to have a

better behavior than that of API 5L X-70 NG, because it has a greater amount of perlite (34.2%)

compared to the ferrite (65.89%), as ferrite is more easily dissolved in the aggressive environment. In

contrast, API 5L X-70 NG possesses 6.10% pearlite and 93.90% ferrite so that it was expected to

display an increase in corrosion rate by the increased presence of this phase [18], which can be

dissolved in a larger volume but not homogeneously. Additionally both steels have the same grain size

so it is assumed that this parameter has no effect on the corrosion rate.

The steel API 5L X-70 T has a greater amount of phosphorus (0.019 wt%), chromium (0.156

wt%) and nickel (0.088 wt%) with respect to X-70 NG, which contains 0.009 wt % of phosphorus,

chromium (0.013 wt %) and nickel (0.005 wt %), but it contains more copper than the traditional (T)

with a 0.286 wt %, which is almost twice of X-70 T, so it appears that Cu in X-70 NG is the

responsible of the improvement in corrosion resistance as a result of the formation of high stability

oxides. As chromium and nickel has the quality to be combined with sulfur to reduce the damaging

effect on the traditional API X-70 (T) and some authors assumed that cathodic reaction in presence of

oxygen contributed to film formation, therefore corrosion resistance of traditional X-70 is derived from

the synergetic effect of chromium and nickel to form an insoluble rust layer at the initial stage of

corrosion as suggested by Choi et al. [19].

3.5 SEM surface characterization

Figure 6 display SEM micrographs from steel surfaces at low magnification (100X). It is

observed that corrosion products have certain similarities; the presence of a uniform layer that is

evident and which eventually overgrows the corrosion products. In micrograph (b) a greater degree of

cracking can be appreciated but not any kind of detachment, feature associated with a dramatic

increase in the corrosion rate as discussed in Figure 6.


2913

Figure 6. Micrographs of steel API 5L X-70 T (a) and API 5L X-70 NG (b) at 100X.

Figure 7. Micrographs of steel API 5L X-70 T (a) and API 5L X-70 NG (b) at 500X.

In Figure 7 the two steel surfaces at higher magnification (500X) are shown, It is observed in

more detail that the corrosion products grown on a first layer, which have the form cactus type; fact

that has already been reported by some authors [20]. Corrosion products seem to be quite porous and

to some point brittle as seen in micrograph (b), where NG steel shows cracks, which appears to be the

result of the material detached because of fluid flow action, thus there may be areas where the metal is

not protected to a certain degree so that corrosion is active.

Figure 8 and 9 show the mapping images of steels API 5L X-70 T and API 5L X-70 NG after

corrosion tests, where O, S and Fe are identified. These elements indicate the presence of a protective

layer of FeO, Fe2O3, Fe3O4, besides some sulfides such as mackinawite (FeS1-x) on the film formed and

corrosion products, as reported in the literature [21-23]. Figure 8 also shows that carbon, iron,

chromium and nickel are distributed all over the analyzed surface, while oxygen and sulfur are

displayed in certain regions in a higher proportion than that of the general distribution. This is an

indication that there exist regions where a greater amount of oxides and sulfides predominated as is the

case for the X-70 NG.


2914

Figure 8. EDX-SEM microanalyses by element after corrosion tests in sour brine for API 5L X-70 T.

Table 3. Chemical composition (wt %) of corrosion scale on API 5L X-70 T.

Element

Line

Weight %

Atom %

C K 11.36 24.53

O K 27.81 45.09

Na K 0.92 1.04

Si K 0.03 0.03

S K 1.89 1.53

Cl K 3.07 2.25

Ca K 0.01 0.00

Cr K 0.17 0.09

Mn K 0.58 0.27

Ni K 0.23 0.10

Cu K 0.00 0.00

Fe K 53.94 25.06

Total 100.00 100.00


2915

Figure 9. EDX-SEM microanalyses by element after corrosion tests in sour brine for API 5L X-70

NG.

Table 3 shows the results obtained by EDX microanalysis for API 5L X-70 T, where the

elements with greater presence are (wt %): C-11.36, Fe-93.94, O-27.81and S-1.89, which suggests that

the combination of these elements generated a larger content of both sulfides and oxides that those

contained in steel X-70 NG (Table 4). Chromium (0.17 wt%) and nickel (0.23 wt%) present indicates

that these elements in combination with oxygen could form the corresponding oxides and sulfides,

which are highly stable and contribute to corrosion control.


2916

Table 4. Chemical composition (wt %) of corrosion scale on API 5L X-70 NG.

Element

Line

Weight %

Atom %

C K 13.95 33.09

O K 16.31 29.05

Na K 1.61 1.99

Si K 0.21 0.21

S K 2.83 2.51

Cl K 0.14 0.11

Ca K 0.16 0.11

Cr K 0.13 0.07

Mn K 0.37 0.19

Ni K 5.43 2.64

Cu K 0.29 0.13

Fe K 58.58 29.89

Total 100.00 100.00

Figure 9 shows the presence of carbon, iron, chromium, nickel and copper distributed all over

the analyzed surface of X-70 NG. Oxygen, sulfur and nickel are distributed on the entire surface

though there are areas with a higher content of these elements. It is observed that oxygen, sulfur,

nickel, chromium and copper seems to be present in a higher proportion that in X-70 T, which is not

true as seen in Tables 3 and 4. It is thought then that the distribution of these elements on steel X-70

NG surface formed more stable compounds forming a tougher film for corrosion control and

consequently seems to appear in a higher concentration on surface when compared to X-70 T.

3.6 XRD Characterization of corrosion products

20 40 60 80 100

1000

1200

1400

1600

1800

2000

2200

2400

2600

*

Fe2O

3 Rombohedral Hematite

Fe2O

3 Tetragonal Hematite

Fe2O

3 Cubic Maghemite

Fe3O

4 Ortorrombic Magnetite

Fe3O

4 Cubic Magnetite

FeS2 Ortorrombic Marcasite

FeS Tetragonal Mackinawite

FeS2 Cubic Pyrite

API 5L X-70 T Brine + 10% kerosene + 1382.7 ppm H2S (Immersion)

Inte

ns

ity

(a

.u)

2degrees)

Figure 10. XRD analysis of corrosion products in API 5L X-70 T after immersion test for 24 hours in

sour brine.


2917

Figure 10 shows the diffraction pattern obtained for the steel API 5L X-70 T after 24 h of

immersion in the corrosive environment. Steel surface displayed a wide variety of corrosion products

to conform a mixture of oxides and sulfides. Crystalline structures with distinct phase were identified,

such as the most predominant orthorhombic Marcasite (FeS2), cubic maghemite (Fe2O3) and tetragonal

mackinawite (FeS). It is also observed that sulfides formed over oxides, which causes an improved

behavior on corrosion resistance.

20 40 60 80 100

0

500

1000

1500

2000

2500

3000

3500

4000

API 5L X-70 NG Brine + 10% kerosene + 1382.7 ppm H2S (Immersion)

Fe2O

3 Cubic Maghemite

Fe2O

3 -Cubic Oxide Iron

Fe3O

4 Ortorrombic Magnetite

FeS Hexagonal Troilite

FeS2 Ortorrombic Marcasite


Inte

ns

ity

(a

.u)

2degrees)

Figure 11. XRD analysis of corrosion products in API 5L X-70 NG after immersion test for 24 hours

in sour brine.

For steel API 5L X-70 NG, the formation of corrosion products on the surface after carrying

out the immersion test for 24 hours is observed in Figure 11. X-70 NG provided a much smaller range

of phases, where predominates oxides on the formation of sulfides. In this case, the phase formed in

the largest amount was identified as cubic maghemite (Fe2O3). It is important to mention that this

phase is protective but to some extent as it has been reported that an oxide is less protective than a

sulfide [24].

20 40 60 80 100

0

5000

10000

15000

20000

25000

30000

35000

API 5L X-70 T Brine + 10% kerosene + 1382.7 ppm H2S

Fe2O


Fe2O

3 Cubic Maghemite

Fe3O

4 Cubic Magnetite


FeS Tetragnal Mackinawite

FeS2 Cubic Pyrite

Inte

ns

ity

(a

.u)

2degrees)

Figure 12. XRD analysis of corrosion products deposited on API 5L X-70 T steel surface after

corrosion tests in a sour brine solution.


2918

In Figure 12, a diffraction pattern obtained for the steel API 5L X-70 T in the corrosive

medium used is observed. In this case it is possible to see that a mixture of oxides with sulfides are

formed with different crystal structures to be the predominant phase that identified as cubic maghemite

(Fe2O3), which also appears in the immersion test.

The two steels formed oxides such as hematite (rhombohedral), maghemite (cubic) and

magnetite (cubic), and also sulphides such as mackinawite (tetragonal), troilite (hexagonal) and pyrite

(cubic and triclinic). Mackinawite is a common mineral composed of tetragonal crystals, whereas

troilite is hexagonal though both are considered protective layers. The different crystal structures of

iron sulfides formed in H2S containing corrosive media were described in detail by D. Rickard et al.

[25]. The crystal structures of the corrosion products in film significantly vary from each other [26],

whereas the differences in the crystal structures of iron sulfide are derived from differences in the

corrosive medium [27].

20 40 60 80 100

0

5000

10000

15000

20000

25000

30000

35000

Fe2O


Fe2O

3 Cubic Maghemite

Fe3O

4 Cubic Magnetite



FeS2 Triclinic Pyrite

API 5L X-70 NG Brine + 10% kerosene + 1382.7 ppm H2S

Inte

ns

ity

(a

.u)

2degrees)

Figure 13. XRD analysis of corrosion products deposited on API 5L X-70 NG steel surfaces after

corrosion tests in a sour brine solution.

The presence of some oxides like Fe2O3 and Fe3O4 [24] partially protects the steel surface from

further dissolutions. It is also noted that oxides predominated much more (peak intensity) than sulfides,

even so there was a better protection for X-70 NG steel, which slightly reduced corrosion rate in

comparison to X -70 T (~4 mm/y). It is further note that the two steels form sulfides, which are

considered protective due to its stability and their crystalline structure as it is the case of hexagonal

troilite, tetragonal mackinawite and pyrite phase. However, for steel X-70 NG where a better

protective behavior was found triclinic pyrite was present (Figure 13), this phase is more compact

compared to cubic pyrite that was formed on steel surface of X-70 T.

4. CONCLUSIONS

Although both steels have different microstructures, X-70 NG has more ferrite to be dissolved,

however this parameter seems no to be a factor that influences corrosion resistance.


2919

The improved performance of X-70 NG may be attributed to a synergistic work of Cr, Cu; Ni

and P. In the case of new generation steel, copper is in approximately twice as much compared to the

traditional steel, this can be combined with decreasing sulfur and their harmful effect and additionally

favors the cathodic reaction where oxygen is formed. Chromium and nickel helps retard only anode

reaction (dissolution of the ferrite).

The presence of copper in the corrosion products is essential for corrosion resistance, as it is

considered that works like chromium and nickel to form highly stable oxides on the steel surface, and

also Cu allowed forming a mixture of oxides and sulfides.

The diversity of corrosion products identified on the surface of each steel is very important

since they together have a protective function, even though the quantitative analysis shows that a

greater amount of oxides (X-70 NG) are formed respect to sulfides (X-70 T), these latter compounds

improved corrosion behavior and are more stable than the oxides and may become dissolved by the

same action of the corrosive medium.

ACKNOWLEDGEMENTS

The authors would like to thank the Consejo Nacional de Ciencia y Tecnología-México (CONACYT)

for sponsorship. Acknowledgments are also due to the Instituto Politécnico Nacional (IPN), Instituto

Mexicano del Petróleo (IMP) and the Group of Pipe Integrity Analysis (GAID) for the facilities

provided and financial aid, respectively.

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effect of flow rate on the corrosion products formed on

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